1. Field of the Invention
This invention relates to the production of flexible polyurethane foams. More particularly, this invention relates to a process for producing low density, flexible polyurethane foams at a low isocyanate index. This invention especially relates to a process for producing conventional, free-rise, low density, flexible polyurethane foams, which exhibit a substantially open cell structure without crushing and without using any inert blowing agents such as chlorofluorocarbons, methylene chloride, or other halocarbons.
2. Description of Related Art
Polyurethane foams are prepared by reacting a polyisocyanate with an active hydrogen-containing compound, as measured by the Zerewitinoff method, such as a polyol, in the presence of water and other optional ingredients. Catalysts are employed to promote two major reactions.
One reaction is primarily a chain extending isocyanate-hydroxyl reaction or gelation reaction by which a hydroxyl-containing molecule is reacted with an isocyanate-containing molecule to form a urethane linkage. The progress of this reaction increases the viscosity of the mixture, and generally contributes to crosslink formation with polyfunctional polyols (i.e. polyols having a nominal functionality above 2). The second major reaction comprises an isocyanate-water reaction wherein an isocyanate-terminated molecule is extended by urea formation and by which carbon dioxide is generated to "blow" or assist in the "blowing" of the foam. The in-situ generation of carbon dioxide by this reaction plays an essential part in the preparation of "one-shot" flexible polyurethane foam. Such foams are often referred to as "water-blown" flexible polyurethane foams.
In order to obtain a good urethane foam structure and desired physical properties, these reactions must proceed simultaneously at competitively balanced rates and in properly balanced degrees relative to each other. For example, if the carbon dioxide evolution (i.e. the water reaction) is too rapid in comparison with the chain extension reaction, the foam tends to split or collapse. Alternatively, if the chain extension reaction is too rapid in comparison with the reaction that generates carbon dioxide, foam rise will be restricted, thus resulting in a higher-density foam with a high closed cell content, i.e. a tight foam.
It has long been known that tertiary amines, such as trimethylamine, triethylamine, dimethylaminoethanol, tetramethyl propanediamine, triethylenediamine, dimethylethanolamine, methyl triethylenediamine, N-methylmorpholine, N-ethylmorpholine and especially bis-(2-dimethylaminoethyl) ether are effective for catalyzing the water-isocyanate reaction that causes carbon dioxide evolution.
However, tertiary amines are only partially effective as catalysts for the chain extension reaction and thus normally are used in combination with other catalysts, typically an organic tin catalyst. For example, in the preparation of flexible foams, a one-step or "one-shot" free rise process has long been used wherein a tertiary amine is employed for promoting the water-isocyanate reaction; while an organic tin compound is used in synergistic combination with the amine to promote the chain extension reaction.
Organic tin compounds particularly useful in making flexible foams from polyether feedstocks include stannous or stannic compounds, such as a stannous salt of a carboxylic acid, i.e., a stannous acylate; a trialkyltin oxide; a dialklyltin dihalide; a dialkyltin oxide, and the like, wherein the organic groups of the organic portion of the tin compound are hydrocarbon groups containing from 1 to 8 carbon atoms. For example, dibutyltin dilaurate, dibutyltin diacetate, diethyltin diacetate, dihexyltin diacetate, di-2-ethylhexyltin oxide, dioctyltin dioxide, stannous octoate, stannous oleate, etc., or a mixture thereof, have been used.
Polyurethane foams are widely employed in the manufacture of a variety of products and, depending on the end use, can be tailor made to fit the particular application and desired physical properties. The polyurethane industry has come to recognize two, generally distinct, categories of flexible foam products: high resilience foams and conventional, lower resilience foams. High resilience (HR) foam is widely used for furniture cushions, mattresses, automotive cushions and padding, and numerous other applications requiring better support and comfort. Conventional foam also is used in these applications and finds additional applications in the areas of carpet underlays and packaging materials.
HR foam is differentiated from conventional foam by its higher comfort or support factor and higher resilience. In its strictest definition, as set forth in ASTM Method D-3770-79, an HR foam is one having a ball rebound value (ASTM D-3574) of greater than 60%, though, in practice, foams with a ball rebound value as low as about 55% are often included in the class of HR foams. The lower resilience, conventional foam typically has a ball rebound value of less than about 55% and often below 50%. HR foam also is usually produced using low water levels to provide higher foam densities, typically above 1.5 pounds per cubic foot (pcf) and often above 2.5 pcf, while conventional foam generally has a density below about 1.8 pounds per cubic foot (pcf), and for the most part below about 1.5 pcf.
HR foam generally is produced using high ethylene oxide content polyols having equivalent weights above about 1600 and primary hydroxyl contents of above 50%; while conventional foams are made using lower ethylene oxide content polyols of equivalent weights below 1300 and often containing only secondary hydroxyls. These foam types also differ on the surface active agents used in their preparation. Conventional foams normally are made with the highly stabilizing polysiloxane-polyoxyalkylene copolymers while HR foams use less stabilizing surfactants, such as dimethylsiloxane oils. Tight, shrinking foams are produced if the highly stabilizing conventional surfactants are used in HR foam formulations.
HR foams also can be distinguished from conventional foams on the basis of their cell structure or porosity. When initially produced, HR foams have an essentially closed cell structure with very low porosity. It is thought that this structure is, in part, a consequence of the use of more reactive polyols and the higher level of crosslinking in HR foam formulations. Because this cell structure impairs the physical properties of the foam, HR foam generally is processed further in a separate crushing or mechanical deformation operation to open the cells. This adds additional cost to the ultimate foam product. Conventional foams, in contrast, typically exhibit an open cell structure and relatively high porosity as they are produced. Thus, there is no need to process the foam to alter cell structure.
Commercially, water-blown flexible polyurethane foams are produced by both molded and free-rise (slab foam) processes. Conventional foam is almost always made using the free-rise process; where as HR foam often is made using closed molds. Slab foams are generally produced more or less continuously by the free-rise process in large buns which, after curing, are sliced or otherwise formed into useful shapes. For example, carpet underlayment is sliced from large buns of polyurethane foam. Molding is utilized to produce, in what is a batchwise process, an article in essentially its final dimensions. Automotive seating and some furniture cushions are examples of employment of the molding process. Slab foam buns produced using the free-rise process tend to be much larger than molded foams. While molded foam objects are normally less than about ten cubic feet in volume, slab foam buns are rarely less than 50 cubic feet in volume. As a result, the preparation of free-rise foams present some unique problems to those in the polyurethane industry.
Commercial, low density and low resilience, free-rise (slabstock) conventional polyurethane foam formulations typically contain (1) a polyether polyol having an equivalent weight below about 1500 and having more than about 50% secondary hydroxyl groups; (2) toluene diisocyanate at an isocyanate index of between about 105 and 120; (3) a highly efficient polyether-silicone copolymer stabilizer; (4) amine and tin catalysts; (5) water between about 2 to about 6 parts per hundred parts polyol (php); (6) an inert blowing agent such as a chlorofluorocarbon, methylene chloride, or other halocarbons to assist foam blowing and/or to cool the foam and miscellaneous other additives such as fillers, flame retardants, and the like. The inert blowing agents also are utilized to soften foams made at densities below about 2.0 pcf. Isocyanate index is the percentage of the calculated stoichiometric amount of isocyanate needed to react with all active hydrogen components in the formulation. Thus, an isocyanate index of 110 means that 110% of the amount of isocyanate stoichiometrically required to react with all active hydrogen compounds is used.
In recent years, processes have been sought by the polyurethane industry for making polyurethane foam products while eliminating, or at least substantially reducing, the amount of inert blowing agents, particularly chlorofluorocarbons (CFCs). CFCs are known to damage the Earth's protective ozone layer, an effect which is expected to lead to a greater incidence of skin cancer and related maladies caused by solar exposure, as well as possible catastrophic climate changes. The U.S. Environmental Protection Agency recently has pushed for a complete phaseout of the use of such ozone-depleting chemicals. Another blowing agent, methylene chloride, also has fallen into disfavor due to concerns about short and possibly longer term health effects. Thus, the current trend is to avoid or minimize the use of such inert blowing agents, if possible, in the preparation of polyurethane foams.
Unfortunately, for the most part, commercial production of low density and low resilience, conventional flexible foam, e.g. at densities below about 1.5 pound per cubic foot (pcf), using the free-rise process requires using such inert blowing agents to lower foam densities and, in part of provide cooling. Attempts at preparing such low density foams at normal isocyanate indexes, e.g., at indexes of about 100 to 120, by using additional water to "blow" the foam via the isocyanate-water reaction, causes foam over-heating and significantly increases the hazard of fire. The hazard of fire is greatly diminished when producing molded foam due to the small volume of the articles produced which facilitates their rapid cooling.
Overheating and load (firmness) in free rise foam might be controlled to some extent by lowering the isocyanate index, and carefully controlling other process parameters such as the catalyst system. However, water-only "blown," conventional, flexible polyurethane foams formulated with "conventional polyols" at isocyanate indexes blow about 100, and particularly below about 90 to 95, often experience foam splitting, i.e. sizeable openings or voids in either or both the surface and interior of the foam. The problem with splitting becomes more severe as the isocyanate index is lowered further to control more carefully the potential for foam overheating and/or to produce very soft foams. Thus, increasing the amount of water used to "blow" the foam, while at the same time lowering the isocyanate index, generally does not represent a practical solution to the CFC problem in free-rise foams.
A soft flexible foam, apparently produced via a free-rise, one-shot process at isocyanate indexes between about 70 and 100 by using polyols (triols) of lower molecular weight (1000) and lower equivalent weight (350), either alone or in conjunction with some standard, i.e. conventional, 3000 molecular weight polyol (triol), up to 4 php water and slightly lower catalyst levels is described by F. J. Dwyer et al. in the Proceedings of the SPI Cellular Plastics 7th Annual Technical Conference. Densities down to about 1.5-1.6 pcf purportedly were produced without auxiliary (inert) blowing agents. However, still lower foam densities required the use of inert blowing agents. T. M. Smiecinski, S. E. Wujcik and O. M. Grace (Polyurethanes 88: Proceedings of the SPI-31st Annual Technical/Marketing Conference, Oct., 1988) also describe the lab scale production of soft foam using a lower molecular weight triol and up to 5 php water, at an 85 isocyanate index. Foam with a density as low as 1.38 pcf purportedly was produced without axiliary (inert) blowing agent.
U.S. Pat. No. 4,833,176 describes a procedure for making a cold cure, soft molded polyurethane foam using an isocyanate index of less than 70, and preferably 40 to 60, water in an amount of up to 15 php as a blowing agent, and optionally a chain extender or crosslinking agent.
Although a variety of methods and polyurethane formulations have been reported in the literature, to date none has disclosed a process of safely making a conventional, free-rise, flexible polyurethane foam at a low density and over a wide range of low isocyanate indexes, i.e. from as low as 60 up to about 95, using conventional polyols while employing little or no inert blowing agent and avoiding problems with foam splitting or foam tightness.